1. Technical Field
This invention relates to pneumatic radial tires for passenger cars and more particularly to such a tire having not only high durability performance but significant weight saving as well.
2. Prior Art
From the points of view of petroleum resource shortages coupled with environmental pollution problems, a keen demand has of late been voiced for fuel-saying motor vehicles. To cope with this trend, great concern has been focussed on the development of lightweight automobile tires. In the case of pneumatic radial tires for passenger cars, the goal of weight reduction or weight saving is expected to be achieved by three options, namely of (1) decreasing the number of steel cords to be counted as reinforcements for a belt layer, (2) minimizing the diameter of each of steel filaments to be applied to the steel cord, or (3) reducing the amount of rubber to be used to form a tread portion which is made relatively large in terms of the volume ratio in the whole tire.
Options (1) and (2) require that the belt layer have a tensile strength at a much higher level than usual. In such instance, a so-called ultrahighly strong steel filament abundant in carbon content is necessarily utilized to fabricate a steel cord of sufficient reinforcement for the belt layer. Although exhibiting good structural strength, higher carbon contents tend to make the resulting steel filament objectionably low in toughness and less resistant to flex fatigue in moist condition. This will lead to broken filament, thus resulting in marred durability of the ultimate tire.
To perform option (3), the tread portion needs to be so designed as to have a smaller gauge of groove bottom to cord surface. The term gauge used here refers to the distance or thickness of rubber from a bottom end of each of a multiplicity of main grooves formed circumferentially on the tread portion to an outer surface of one of steel cord arrays positioned nearer to the groove bottom. Conventional radial tires are usually in the range of 3.0 to 4.5 mm in that gauge and hence rather heavy in weight. In general, such a tire is susceptible, in the region of groove bottom ends, to intensely localized strains during running, which strains are responsible for broken steel cords and impaired tire durability. This adverse tendency can be said to become more noticeable when the gauge of groove bottom to cord surface is set at below 3.0 mm. Much poorer durability is particularly true of the case where the above steel filament of ultrahigh structural strength is employed.